1. Field of the Invention
The present invention relates to packet-based video networks.
2. Description of the Prior Art
It is known to link video and audio devices in a television studio together using a switching device, typically a cross point switch.
A system has been proposed which links audio and video devices in a studio by a packet-switched local area network, for example an Ethernet network, operating with a known protocol such as Internet Protocol (IP). However most, if not all, current video and audio equipment used in studios is not equipped to operate with such a switched network operating according to such a protocol.
The audio and video devices used in a studio include cameras, editors, audio mixers, video tape recorders (VTRs) and play-out switches amongst other examples. It is also known to use monitors to view video which is being played out or to preview on monitors video which is available to be played out.
One of the obstacles to providing circuit-switched studio functionality in a packet-switched network is that video and audio signals have stringent timing and synchronisation requirements. Typically, a video source will be locked to at least two timing signals, a bit clock and a frame synchronisation clock. Copending applications 0204242.2 and 0307459.8 deal with techniques to allow synchronisation of both individual data bits and video frames over a packet-based network. In particular, at a video source node on the network, a video signal is synchronised to reference clock signals. At a receiving node elsewhere on the network, a local clock is synchronised to the reference clock signals, and video data packets received from the source node are handled in synchronism with the local clock.